People who watch television for three hours a day or more are twice as likely to die in the next few years as people who watch little or no TV, a new study has found.

It’s the latest in a series of studies that show sitting still can kill you. But this one has a few twists. TV watching seemed deadlier than sitting at a desk or driving a car all day — and the effect was seen in relatively young, healthy affluent people. So it doesn’t appear that people were watching TV because they didn’t feel well.

“Our findings suggest adults may consider increasing their physical activity, avoid long sedentary periods, and reduce television watching to no longer than one to two hours each day," said Dr. Martinez-Gonzalez of the University of Navarra in Pamplona, Spain.

Martinez-Gonzalez and colleagues used an ongoing study of 13,284 people with an average age of 37. They watched less TV on average than most Americans — 1.6 hours a day, compared to an average of 2.8 hours a day for working Americans measured by the U.S. Bureau of Labor Statistics.

The Spanish volunteers spent an average of two hours a day on the computer and just about an hour a day driving. They were followed for just over eight years.

“Participants reporting three or more hours a day of television viewing had a twofold higher risk of mortality than those reporting less than one hour a day,” Martinez-Gonzalez and colleagues wrote in the Journal of the American Heart Association.

The volunteers in the study seemed healthier than the average comparable Spaniard — over the eight years, only 97 died, while 128 people of similar age, sex, income and so on would have been expected to. Of those who died, 19 died from heart disease or stroke, 46 died from cancer and 32 died of something else.

“Participants reporting three or more hours a day of television viewing had a twofold higher risk of mortality."

For every two extra hours of watching TV over and above one hour a day, the volunteers were 44 percent more likely to die from heart disease or stroke, 21 percent more likely to die of cancer and 55 percent more likely to die from something else, and that’s taking in account their age, sex, whether they smoked, whether they were obese and whether they ate a healthy, Mediterranean diet.

This fits in with other recent research that shows for every two hours spent sitting in front of the computer or television, the average American raises his or her risk of colon cancer by 8 percent, of endometrial cancer by 10 percent and of lung cancer by 6 percent.

Why would that be? “No one yet knows the answer, but if you're the type of person who has a lot of time but spends it sitting in front of the television, you also have other sedentary behaviors, like not going outside too much or engaging in physical activity,” said Dr. Nieca Goldberg, a cardiologist at New York University who was not involved in the study.

Martinez-Gonzalez thought it might be because people tend to binge on junk food when watching TV, but even when they took that into account, TV watching seemed especially deadly.

Animal studies suggest that activities like watching TV might affect how the body manages cholesterol or carbohydrates, or it might raise inflammation — which is linked with cancer and heart disease.

“Compared to driving a car or doing work on a computer, television is a very passive activity,” Goldberg said. Other studies have suggested that watching television lowers the metabolism than even sitting and doing nothing.

There’s an easy fix, however — less TV and more exercise.

"Compared to driving a car or doing work on a computer, television is a very passive activity."

“There are lots of ways you can put physical activity into your daily life like taking stairs instead of the elevator, walking to work” Goldberg said.

“Because I live in New York City, I walk to work,” she added. “Many people can't do that, but I suggest if you're driving to work, park further away in a parking lot so you end up walking a longer distance to the entrance.”

Some other studies suggest that adding exercise isn’t enough — total time sitting can cause obesity and other disease even if people hit the gym, but breaking up the day would help, Goldberg said.

“During lunch hour, why not take a walk around the block? The American Heart Association recommends at least 150 minutes per week of moderate aerobic activity,” she said.

“Walk to the store, spend time walking around the mall. Take stairs instead of the elevator. Go outside and get a walking group together.”